Fluid flow sensors measure the rate at which a fluid flows past the sensor. There are many different types of fluid flow sensors. Ultrasonic flow sensors use a frequency shift caused by the flowing fluid to measure the fluid flow rate. Electromagnetic flow sensors measure fluid flow rate induced frequency shifts of electromagnetic waves such as radio waves, microwaves, or light waves. Thermal fluid flow sensors are based on the heat transfer effects of a flowing fluid. One type of thermal fluid flow sensor is based on measuring the amount of heat energy carried away from a heated element. Another type of thermal fluid flow sensor measures the temperature difference between a temperature sensor upstream of a heated element and the temperature downstream of the heated element. Those practiced in the art of flow sensing are familiar with thermal fluid flow sensors.
Chemometrics is a set of mathematical techniques wherein single measurements or multivariate measurements can be used to produce compensated measurements or to identify a chemical analyte. Software packages for chemometrics are available so that a processor, such as a computer CPU, can use chemometrics. Those practiced in the arts of linear regression, applied chemistry and chemical signature analysis are familiar with chemometrics.
The physical properties of a fluid can cause a fluid flow sensor to produce incorrect readings. For example, a thermal fluid flow sensor depends on the thermal conductivity of the fluid. Two fluids with the same flow rate but different thermal conductivity can yield different flow rate measurements. Other physical properties that can affect fluid flow measurements are electrical conductivity, pressure, and density. Fluid flow sensors should be calibrated in order to produce accurate fluid flow measurements.
Two types of calibration are sometimes used for fluid flow sensors. One type is deployment-based calibration and the other is maintenance-based calibration. In deployment-based calibration, a programmed calibration is chosen based on the where the fluid flow sensor is going to be used. For example, if the fluid is alcohol, then the fluid flow sensor is calibrated for alcohol. In maintenance-based calibration, the output of the fluid flow sensor is exposed to known fluid flow rates and its output is adjusted to match those known fluid flow rates.
The problem with deployment-based calibration is that the calibration is set for a reference fluid that was measured some time in the past. However, the physical properties of the reference fluid in the past can be different from the physical properties of the actual fluid in the present. One reason is that sometimes the reference fluid was not measured under ideal conditions. Another reason is that the actual fluid being measured is rarely flowing under ideal conditions.
The problem with maintenance-based calibration is that the calibration is often performed poorly and that the fluid properties can change. To be useful, the calibration must be performed correctly and regularly by trained, motivated, and properly equipped individuals. Such individuals are often not available. Fluid properties can change because the environment changes or because the fluid itself changes. Environmental changes as minor as the weather or the setting of the sun can cause fluid properties to change. The fluid itself can change intentionally through human intervention or accidentally through contamination.
A fluid flow sensor that can produce accurate measurements regardless of the changing physical properties of the fluid is needed.
The embodiments discussed herein directly addresses the shortcomings of the prior art by utilizing chemometric analysis to compensate for the effects of changes in a fluid's physical properties on fluid flow measurement.